fix: address skeleton/nav-progress review findings

Correctness:
- MangaGridSkeleton: covers rendered 0px tall because height="0" is a
  definite height that defeats aspect-ratio. Skeleton gains an `aspectRatio`
  prop (height stays auto) so the 2/3 cover box is reserved and the grid no
  longer shifts when real covers load.
- Move `.manga-card` layout into global tokens.css (it was scoped to
  MangaCard, so the skeleton's cards inherited none of it). Both consumers
  now share one definition.
- NavProgress: rebuilt as an idle/loading/done state machine. It now
  completes to full and fades on settle (was snapping away instantly),
  restarts the trickle when one navigation supersedes another (was parking
  frozen near 90%), animates transform/opacity only with will-change gated to
  the active phase, and hides itself in reader fullscreen.
- Skeleton: text-variant height resolved in one place (no inline style
  shadowing a stylesheet rule); shimmer gradient/animation gated behind
  prefers-reduced-motion: no-preference so reduced motion shows a flat block.

A11y:
- Home loading region announces via visually-hidden text instead of an
  aria-label a live region won't reliably read.

Cleanup:
- Drop the fragile :first-child selector and index-keying ceremony in
  MangaGridSkeleton; simplify Skeleton's style construction.

Tests: adds regression guards for the 0px cover collapse, the aspect-ratio
box, text-height defaulting, and the nav-progress complete/restart phases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/tokens.css

@@ -349,6 +349,20 @@ img {
     gap: var(--space-4);
 }
 
+/* A single cover cell — the catalog/author/collection cards and the
+   loading skeleton share this so their layout can't drift. */
+.manga-card {
+    display: flex;
+    flex-direction: column;
+    gap: var(--space-2);
+    list-style: none;
+    /* Grid items default to min-width: auto which equals the intrinsic
+       content size — long author / title strings then push the cell past
+       `1fr`. Forcing 0 lets the column control the width and the children
+       ellipsize inside it. */
+    min-width: 0;
+}
+
 @media (max-width: 640px) {
     .manga-grid {
         grid-template-columns: repeat(4, minmax(0, 1fr));